Perf: Debounce script `updateProjectOptions` (fires on every caret move)

## Description

In `FSharpProjectOptionsManager.fs`, `onChangeCaretHandler` calls `updateProjectOptions` on every caret position change. This in turn calls `checker.NotifyFileChanged(document.FilePath, scriptProjectOptions) |> Async.Start`, asking FCS to re-evaluate script options, potentially triggering a new parse and `#r`/`#load` resolution on every keystroke/caret move.

## Call chain

User moves caret → `IVsTextViewEvents.OnChangeCaretLine` → `onChangeCaretHandler` → `updateProjectOptions` → `Async.Start(checker.NotifyFileChanged(...))` → FCS script options resolution.

## Fix

Debounce `updateProjectOptions`: use a `CancellationTokenSource` that is swapped/cancelled on each call and only actually notifies after a 500ms idle period, implemented as a `cancellableTask` using `Task.Delay(500, ct)`.

## Priority

P2 (medium-term)

Start in FSharpProjectOptionsManager.fs at onChangeCaretHandler and follow its call to updateProjectOptions and checker.NotifyFileChanged. Read the existing cancellableTask usage and Async.Start flow first. Done means repeated caret changes cancel pending work and notification occurs only after 500ms without another change.

Written by the indexing model from the issue text.